In echo depth measuring in oceanography, a pulse of energy in the form of a short burst of an audio or ultrasonic tone frequency is sent from a transducer, mounted on a vessel or ship, toward the bottom of the ocean or other body of water. The tone or energy pulse is reflected from the bottom, back toward the vessel where that echo is received by the same or another transducer.
The speed of travel of a tone pulse through ocean water at various temperatures and depths is reasonably accurately known. It is also known that the transit time of such a pulse from the vessel to the bottom and back to the vessel is directly related to the depth of the water under the vessel-mounted transducer.
In order to make gross measurements, e.g., to determine whether the depth at a particular point is a hundred feet or a thousand feet, occasional pulses widely spaced in time are quite adequate. In relatively deep water it may take as much as eight or ten seconds for an echo to return to a vessel-mounted transducer. However, if a ship is traveling at a speed of fifteen knots during the transit period of the pulse and its echo, the vessel may travel as much as 200 to 400 feet before the next pulse of energy is transmitted. This is perfectly acceptable when one is only interested in avoiding an accidental grounding of the vessel.
However, for oceanographic survey, such, for example, as mineral exploration or cable laying, greater accuracy is required than is possible in a system in which sounding pulses are transmitted every 200 feet or more. This usually requires that the ship travel slower, which seriously escalates time and cost required to make a survey, Alternatively, in shallower water, the pulses can be somewhat more frequent. However, in deeper water, where it takes as much as ten seconds for the sounding pulse to travel from the ship to the bottom and for its echo to return to the ship again, soundings at an interval so frequent as to transmit a pulse before receipt of the echo of the prior pulse would tend to confuse the depth recorder.